1. Field of the Invention
This invention relates to an automatic focusing device in which distances are measures by an auxiliary light illuminating an object to be photographed and in which a lens is shifted to an in-focus position by driving the lens according to the distances measured.
2. Description of the Prior Art
In order to keep a lens in focus while following a moving object to be photographed, an automatic focusing device must be arranged to repeat its distance measuring action at predetermined intervals.
An ideal automatic focusing device is preferably usable under any condition. Actually, however, it becomes impossible to compute a focusing condition when an object to be photographed is excessively dim. To solve this problem, one may arrange for an auxiliary light source to emit light every time an image signal is received, i.e., at every distance measuring action.
Normally, once an in-focus state attained, a focusing action is not required until the object move. After the in-focus state is obtained, therefore, it is possible either to stop emission of the auxiliary light or to extend the interval between one distance measuring action and another. Conventional devices are arranged to measure the distances after the lenses are in-focus at the same interval as before the lenses attain the in-focus state Accordingly, the auxiliary light source consumes excessing electric energy.